Gammaherpesviruses are ubiquitous persistent pathogens that infect a substantial majority of the world's population. Infection does not have deleterious consequences in most healthy individuals, due to the presence of a potent immune response that contains the infection. In AIDS patients and others with suppressed immune systems, breakdown in immune surveillance can be accompanied by life-threatening gammaherpesvirus- associated disease. There is a pressing need to understand the mechanism by which the immune response, particularly the CD8 T cell response, maintains optimal control of the infection. Our previous work, using the murine gammaherpesvirus model system, clearly indicates that the differentiation state of memory CD8 T cells is a key determinant to maintain benign levels of persistent virus. Memory T cells can be categorized into two types - effector memory T cells (TEM), with the capacity for immediate effector function but limited proliferation, and central memory T cells (TCM) with delayed effector function but the capacity for extensive proliferation. TEM predominate in persistent infections, and are likely critical for surveillance of the virus. However the factors that determine cell fate toward TEM or TCM lineage are currently obscure. We have recently discovered that CD8 cells lacking a key microRNA fail to differentiate appropriately into effector cells, instead skewing toward the memory phenotype. These cells differentiate poorly into long-lived memory cells, and have a marked skewing towards TCM, whereas wild type cells predominantly assume the TEM phenotype. This is important because it indicates a pivotal role for this microRNA in immune surveillance to MHV-68. In addition, factors controlling TCM differentiation are very actively sought, as the generation of these cells is the 'holy grail' of vaccine design against many infectious agents and tumors. This presents a unique opportunity to determine the functional role for microRNA regulation of antiviral CD8 T cell responses. In this proposal we address three hypotheses: (i) During the acute phase of the infection, there is a deficit in the production of short-lived effector CD8 cells, but the generation of memory precursor cells is intact. (ii) TEM fail to form correctly but differentiation to TCM is intact. (iii) Control of MHV-68 infection is defective in the absence of microRNA, for two reasons. Firstly the T cell response is unable to control the infection. Secondly B cell colonization by latent MHV-68 is altered in the absence of microRNA. Our knowledge of the functions of microRNAs increases each year, as do the tools that enable us to manipulate their activity. Therefore the major impact of this work will be to illuminate the roles of microRNA in the antiviral immune response, which have the potential to be manipulated to effect better control of medically important pathogens such as the gammaherpesviruses.